Improvements in computing and information technologies have led to an increase in unauthorized or illegitimate use of network resources. Such illegitimate use can be harmful to the network resources or to the business interests of the parties that own the network resources. For example, a search engine may expend considerable time and effort to generate a search index. So-called search spammers may set up automated processes (sometimes called “bots”) to replicate or otherwise tamper with the search index by sending multiple searches to the search engine. In addition to replicating data, illegitimate users of the network resources may cause other problems, such as using up bandwidth or other processing resources or generating misleading data (e.g., skewing popularity data).